Different types of monofrequency microcrystal lasers are known. A feature of such lasers is the formation of one or more longitudinal modes of the light wave field in the laser resonator. The number of possible longitudinal modes is defined in this way by the optical path length of the resonator, in which case a sufficient optical path length, one mode or a plurality of modes can develop. If the optical path length of the resonator is reduced, the maximum possible number of laser modes is reduced. The smaller the optical path length becomes here, the larger is accordingly the frequency spacing between two adjacent modes. The possible oscillating modes of such a laser are the ones which are subjected to amplification. In order to be subjected to amplification, the modes must lie within the amplification bandwidth of the laser medium (laser-active medium). Accordingly, with decreasing resonator length, the number of amplified modes decreases. The number can also become zero in the case that the absolute position of the modes lies outside of the amplification bandwidth of the laser medium. With decreasing resonator length, the period and the pulse duration of the selected laser pulse decreases at the same time, and in which laser pulses with pulse durations<1 ns up to <50 ps can be obtained.
By a suitable selection of the optical path length of the laser resonator it is possible that the oscillating laser mode lies in a defined manner within the amplification bandwidth, for example in the maximum, and thus is subjected to an amplification as high as possible. Accordingly, the output power of a laser is at a maximum if the optical path length is actively adapted to the relative position of the oscillating mode in the maximum of the amplification bandwidth of the laser medium.
An amplitude fluctuation of a microcrystal laser in the uncontrolled range is approximately 6% and thus is not suited for applications which require a lower amplitude fluctuation from pulse to pulse. A stabilization of the optical path length of the resonator counteracts an amplitude fluctuation through smaller differences in the amplification by centering the position of the emission wavelength in the maximum of the amplification bandwidth of the laser medium.
From DE 43 06 919 C2, a method for stabilizing the optical path length of a resonator is known. This publication discloses a microcrystal laser of the relevant type for generating laser pulses which has a laser resonator which has a laser medium (laser-active medium) arranged between two mirrors. The known microcrystal laser further has an arrangement for stabilizing the optical path length of the laser resonator.